Up to now there have been known optical fibers composed of inorganic glass which are superior in light transmitting properties over a wide range of wavelengths. These inorganic fibers, however, exhibit inferior processability, poor flexibility, and moreover they are expensive. Hence, optical fibers based on plastics have been developed. These plastic optical fibers are obtained by producing fibers having core-cladding structure wherein the core material is a polymer having a high refractive index and a high light transmittance and the cladding material is a polymer having a lower refractive index than that of the core material and being transparent. For useful polymers as core components having high light transmittance, amorphous materials are desirable and polymethyl methacrylate, polycarbonate, and polystyrene are generally used. Of these polymers, polymethyl methacrylate, superior in transparency and furthermore in mechanical properties and weather resistance, is used on an industrial scale as the core material of high-performance plastic optical fibers. However, the glass transition temperature (Tg) of polymethyl methacrylate is 100.degree. C. and the use thereof has been limited in the aspect of heat resistance.
Moreover, various plastic optical fibers comprising polycarbonate as the core material have been proposed, but they are not put into practical use, since the light transmission loss through polycarbonate is large and no cladding material superior in heat resistance has been developed.
Therefore, it is proposed, for example, in Japanese Patent Application Laid-Open No. 18608/83, to enhance the heat resistance by forming a three- or more-layer structure wherein a protective layer or the like having the same composition as that of the core material is provided additionally around the cladding.
In spite of such structural improvement, a large amount of thermal shrinkage is citable as a drawback common to the conventional optical fibers of plastics. It is the present situation that improvement in this respect is insufficient. In consequence, their light transmission characteristics are markedly deteriorated on account of their thermal shrinkage, for instance, when they are used as optical communication means or photosensors which are set in high-temperature sites such as engine rooms of automobiles or of ships, so that they have the disadvantage of the application thereof being restricted.
In case of an optical fiber of three-layer structure as stated above, the action of depressing thermal shrinkage can be enhanced by reducing the core diameter and relatively thickening the protective layer. In this case, however, there is caused a problem that the core diameter needs to be extremely small, for example, about 100 .mu.m, such fibers are difficult to connect one with another, and when the fiber is combined with a light emitting diode (LED), the light incidence efficiency will be lowered.
Under such circumstances, the present inventors have found that the prior art problems noted above can be solved by using a protective layer of high heat distortion temperature for an optical fiber of threelayer structure and optimizing the diameter or thickness proportions of the component layers. Thus the present invention has been accomplished.